U.S. regulators accused Fiat Chrysler Automobiles NV of using software on its diesel-powered Jeep Grand Cherokee SUVs and Ram pickups that allowed them to spew illegal amounts of pollution into the air, the latest broadside from the government over emission standards.

The Environmental Protection Agency, with little more than a week before President Barack Obama leaves office, delivered a violation notice to Fiat Chrysler accusing it of using illegal software in 104,000 vehicles. The accusation could cost the company $4.63...